The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus. More particularly, the present invention relates to a developing device for an image forming apparatus of the type including a developer carrier, which performs endless movement to convey a developer deposited thereon in the form of a layer, and a regulating member for regulating the thickness of the layer.
In a developing device of the type described, a regulating member is often implemented as a stationary doctor blade contacting or facing a developing roller or similar developer carrier, which performs endless movement. The doctor blade regulates the thickness of a developer deposited on the developer carrier in the form of a layer, so that the developer carrier conveys a preselected amount of developer to a developing position where it faces an image carrier. This successfully stabilizes image density.
The problem with the developing device using the doctor blade is that paper dust and other impurities, as well as deteriorated developer particles, form lumps and are caught in a gap between the developer carrier and the doctor blade. This gap will be referred to as a regulating position hereinafter. The impurities caught at the regulating position form stripe-like grooves in the developer layer deposited on the developer carrier, causing stripe-like irregularities to appear in the resulting image.
To solve the above-described problem, Japanese Patent Laid-Open Publication No. 10-104945, for example, discloses a developing device using a rotatable doctor roller as a regulating member. By rotating the doctor roller while causing the developer carrier to perform endless movement, it is possible to remove the impurities staying at the regulating position and therefore to reduce irregular development. The doctor roller, playing the role of a regulating member, has a surface roughness Rz of 2 xcexcm to 100 xcexcm.
As for a developing device, two different types of developing systems are available, i.e., a contact type developing system and a non-contact type developing system. In a contact type developing system, a developer deposited on a developer carrier and an image carrier, which faces the developer carrier, contact each other such that the developer deposits on a latent image formed on the image carrier. In a non-contact type developing system, the developer on the developer carrier is spaced from the image carrier and deposits on the image carrier by flying away from the developer carrier. Generally, the contact type developing system advantageous over the non-contact type developing system in that it enhances the sharpness of an image, i.e., implements high resolution.
We conducted a series of experiments by applying the contact type developing system to the developing device taught in the previously mentioned Laid-Open Publication No. 10-10495. The experiments showed that irregular development occurred in the form of fine stripes. Such irregular development was particularly conspicuous when toner having a relatively small volume mean particle size of 5 xcexcm to 9 xcexcm was used as a developer in order to enhance resolution. Although this kind of irregular development was less noticeable than the irregular development ascribable to the impurities, it had critical influence on image quality. Extended researches and experiments showed that the above irregular development was ascribable to the following cause. When the roller, serving as a regulating member, had a relatively great surface roughness Rz of 2 xcexcm to 100 xcexcm, fine irregularities existing on the surface of the roller formed fine stripes on a developer layer. In the non-contact type developing system, such stripes do not noticeably effect the deposition of the developer on a latent image because the developer flies away from the developer carrier. In the contact type development system, however, the stripes formed in the developer layer, which directly contacts the latent image, noticeably effects density and brings about irregular development.
Another problem with the doctor roller or similar movable regulating member is that it brings about irregular development due to shape errors. As for the roller, for example, it is almost impossible to practically obviate shape errors on a production line. In practice, the roller has, e.g., a cross-section slightly different. from the expected circular cross-section. As a result, the locus along which the surface of the roller moves is not circular and causes the distance between the surface and the developer carrier and therefore the thickness of the developer layer to vary in accordance with the rotation angle of the roller. This makes the thickness of the developer layer irregular and brings about irregular development. This is also true with a developing device using any other movable regulating member, e.g., one having a semicircular cross-section whose curved surface faces a developer carrier and moves back and forth within the range in which it faces the developer carrier.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 7-295363, 8-227224, 9-319208, 10-10863 and 11-125931.
It is therefore an object of the present invention to provide a developing device capable of reducing irregular development ascribable to impurities caught at the regulating position, compared to the case wherein a doctor blade is used as a regulating member, and obviating fine stripes ascribable to stripes formed in a developer layer present on an image carrier.
It is another object of the present invention to provide a developing device capable of reducing irregular development ascribable to impurities caught at the regulating position, compared to the case wherein a doctor blade is used as a regulating member, and obviating irregular development ascribable to the shape errors of the regulating member.
In accordance with the present invention, a developing device for an image forming apparatus includes a developer carrier performing endless movement while carrying a developer containing toner and an additive covering the toner thereon, and a regulating member for regulating the thickness of the developer deposited on the developer carrier in the form of a layer. The additive of the developer has a particle size of less than 2 xcexcm while the regulating member has a surface roughness Rz greater than or equal to the particle size, but smaller than 2 xcexcm, and performs the endless movement or moves back and forth along a preselected path.
Also, in accordance with the present invention, a developing device for an image forming includes a developer carrier performing endless movement while carrying a developer containing toner and an additive covering the toner thereon, and a regulating member for regulating the thickness of the developer deposited on the developer carrier in the form of a layer. The regulating member has a surface roughness Rz of 1.2 xcexcm or above, but smaller than 2 xcexcm, and performs the endless movement or moves back and forth along a preselected path.
Further, in accordance with the present invention, a developing device for an image forming apparatus includes a developer carrier performing endless movement while carrying a developer containing toner and an additive covering the toner thereon, and a regulating member for regulating the thickness of the developer deposited on the developer carrier in the form of a layer. The regulating member includes a surface layer having a tensile elongation ratio of 150% or above and an under layer having an Ascar C hardness of 90 degrees or below and has a surface performing endless movement or moving along a preselected path.